Space-Based Ground Moving Target Tracker Revealed

tracking moving targets
The Space Force plans to use satellites to keep track of moving targets on the ground, replacing the battle management crew on the E-8C J-Stars.
Credit: Tech. Sgt. Rey Ramon/U.s.Air force

The U.S. Space Force has revived a decades-long dream of launching satellites with radars that can track moving vehicles on the ground.

The new capability would help the Air Force replace a capability now performed by the Northrop Grumman E-8C Joint Surveillance Target Attack Radar System (J-Stars) fleet. The E-8C, which is scheduled for retirement in 2025, carries the APY-7 synthetic aperture radar, which has a ground moving target indicator (GMTI) mode.  

  • U.S. Air Force Rapid Capabilities Office began risk reduction in 2018
  • Space Force plans to partner with commercial companies

“The Space Force has a program where we’re building GMTI from space,” Gen. John Raymond, the service’s chief of space operations, said May 12 at the McAleese Defense Programs Conference in Washington.

The Air Force Rapid Capabilities Office began the program in 2018, identifying technologies and risk reduction efforts, according to spokeswoman Col. Catie Hague. “The program office has been focused on managing risk, while meeting a schedule accommodating the planned retirement of the [U.S. Air Force’s] J-Stars,” she says. 

“Space-based GMTI will provide another way to harness data from the space domain and incorporate it into the secure cloud environment, underpinning the Advanced Battle Management System to sense, make sense and act faster than our adversaries,” Hague says. 

The Space Force opted to declassify the existence of the still-developmental satellite effort just as combatant commanders from U.S. Indo-Pacific Command (Indo-Pacom) are asking Congress to support it. Indo-Pacom commanders have pledged to add $2.3 billion to the organization’s unfunded priorities list for a Space-Based Persistent Radar that can conduct the GMTI and airborne moving-target indication missions (AW&ST March 22-April 4, p. 22)

And Indo-Pacom has discussed a vision for partnering with commercial companies to establish a GMTI capability in space. The Space Force is also planning to turn to commercial industry, where advances in space-based radars appear to be making the change possible. 

“We’re going to leverage commercial more than we’ve done in the past,” Raymond said. “We’re going to try to have a strong partnership between our force and commercial [companies].”

The task of detecting moving targets on the ground is daunting. It requires a satellite moving at orbital speed to track comparatively slow-moving targets on the ground against background clutter at great distances.

The Defense Department and the intelligence community have made several attempts since the late 1990s to develop a space-based radar with a GMTI mode, including the canceled Discoverer II and Space Radar programs. 

A paper on space-based radar physics provided to Aviation Week highlights the technical challenges.

By definition, a space radar is significantly farther away from the target area than an airborne radar operating below 45,000 ft. (13.7 km/8.5 mi.). The greater distance requires either a larger antenna, more power, a higher pulse rate or some combination of all three. 

For a satellite in low Earth orbit (LEO) at around 500 km, the difference is a factor of 25.5. That means the E-8C’s radar antenna needs to be only 4.43 m2 (48 ft.2) at a 45,000-ft. altitude to achieve the same signal-to-noise ratio as a 113 m2 antenna in LEO at 500 km, assuming both share the same power output and pulse rate.

The difference expands significantly at altitudes above 2,000 km in medium Earth orbit (MEO), rising to a factor of 4,223. In theory, that would translate to an antenna 18,707 m2 in area. 

By combining increases in antenna size, power output and pulse rate, however, space radar designers have been able to achieve impressive results over the past decade in orbital experiments. Although using an antenna measuring 22.5 m2, Canada’s Radarsat-2, for example, tracked cars moving as slow as 8.3 kph (5.2 mph) in 2012. China’s Gaofen-3 satellite, meanwhile, also experimented with a GMTI software mode during a 2016 orbital test. 

As a military capability, a single satellite at LEO or MEO altitudes compares poorly to the persistence of an E-8C, which can fly a 9-hr. mission. At orbital speeds, satellites remain over an operational theater-sized area for only minutes, leaving huge gaps in coverage that pop-up threats could exploit. 

To achieve persistence from space, a constellation of satellites is necessary. Capella Space, a commercial provider, is, for example, planning to launch a constellation of 36 microsatellites to provide global coverage with synthetic aperture radars. But Capella has not advertised a GMTI capability as a commercial service and its constellation is not designed to provide seamless global coverage anyway.

Steve Trimble

Steve covers military aviation, missiles and space for the Aviation Week Network, based in Washington DC.